1. Field of the Invention
The present invention concerns a rolling bearing used for automobiles, agricultural machines, construction machines and iron and steel equipment and, in particular, a rolling bearing which can be applied to bearings for roller followers in engines requiring long life and high seizing resistance and a tapered roller bearing requiring high PV values.
2. Description of the Art
For rolling bearings, various kinds of alloy steels, such as ball bearing steels No. 2 (SUJ-2), have been employed. For such steels, since the surface pressure loaded on ball bearings has tended to increase in recent years along with an increase of load and a reduction of size, an improved endurance life is required for a rolling bearing.
As countermeasures for improving the life of a rolling bearing, optimization of the concentration of retained austenite on a raceway surface or a rolling contact surface, reduction of non-metal impurities and the like have been conducted. The life of the rolling bearing can be extended to some extent by such countermeasures, but they are not still enough to satisfy recent demands for improving the life, particularly, of rolling bearings used for automobiles, agricultural machines, construction machines and iron and steel equipment, ball bearings for roller followers in engines, as well as tapered roll bearings requiring high PV values.
For improving the fatigue resistance of steels, there has been known a method of increasing the hardness of steels. However, in order to increase the hardness of a rolling member, a great amount of carbide-forming elements, such as Mo W and V, have to be added as in a case of high speed tool steels. As a result, since large carbides crystallized upon solidification to act as initiation points for fatigue, long life cannot always be attained for the rolling bearings.
On the other hand, there has been known a prior art method for obtaining highly tough and abrasion resistant parts by applying ion carburization to steels containing 3 to 15% by weight of Cr, as described in Japanese Patent Laid-Open Hei 1-201459 and Hei 1-234554.
However, since no consideration has been made in this prior art for applying the steels to the rolling bearings, there has been a problem that no distinct teaching is obtainable for the range of hardness of steels and the size of carbides contained in the steels which are important relative to the rolling fatigue resistance, the relationship between a rolling element and a bearing ring which is important relative to the life of the rolling bearing, as well as the average amount of retained austenite which is important relative to the dimensional stability of the rolling bearing.
A prior patent application filed on behalf of the Assignee of the present application relates to an invention directed to an alloy steel containing C: 0.3-0.5, Cr: 3-14 (each % by weight), Ti: 40 ppm and 0: 12 ppm, applied with carburizing or nitriding heat treatment, and containing 20 to 50% by weight of fine carbides with a size preferably of 0.5 to 1.5 .mu.m and 10 to 15% by weight of retained austenite at the surface, in which stress concentration is moderated by the retained austenite thereby moderating the rolling load concentrated on the edges of dents caused by obstacles in a lubricant and to thereby prevent the occurrence of microcracks (refer to U.S. patent application Ser. No. 07/557,574). In the prior application, reduction of the mechanical property of a member caused by the increase of the retained austenite at the surface is compensated by the amount of the carbides at the surface and, as a result, long life can be maintained even under lubrication with a lubricant containing obstacles. However, the method disclosed in this prior invention is quite different from that of the present invention.